In the fabrication of semiconductor integrated circuits (ICs), various layers of inorganic material form semiconductor-based substrates. Some of the inorganic layers are patterned in desired shapes. The resulting inorganic layers form individual devices and interconnect structures within the IC.
Patterning conventionally includes masking an underlying layer with an organic resist material, such as photoresist, exposing the resist, and removing exposed areas of the mask to form a patterned mask layer. The exposed inorganic layer underlying the patterned mask layer is then removed using an appropriate etchant. The patterned mask layer is then removed.
Etching is a process for removing unwanted material (i.e., partial or complete layers of material) from a surface (e.g., the surface of a semiconductor-based substrate). Organic or inorganic material, which may be patterned or unpatterned, of a substrate surface can be removed using an etching technique. Ideally, etching should precisely remove material that is not covered by a patterned mask layer (i.e., material that is "exposed" when a patterned mask layer is used).
The etchant is typically chemically varied according to the type of material being etched. Etchants are characterized as isotropic or anisotropic. Isotropic etchants remove material in all directions at the same rate. Anisotropic etchants do not remove material in all directions at the same rate. Etchants are further characterized as being selective or non-selective, depending on their ability to differentiate between material that they effectively etch. Selective etchants remove different types of material at different rates.
Etching can occur in a wet or dry processing environment. Wet etching refers to the contact of a substrate surface with a liquid chemical etchant. Material is removed as an agitated liquid or spray, for example, passes over the substrate surface. Dry etching refers to the contact of a substrate surface with a gaseous plasma etchant. Wet etching is preferred over dry etching due to its ability to provide high, reliable throughput with excellent selectivity. One problem associated with conventional wet etching, however, is that etched material constituents may move within etched or partially etched openings on the substrate surface. Thus, all of the etched material constituents are not removed from the substrate surface. Furthermore, oftentimes etchants do not completely wet a substrate surface, resulting in incomplete or nonuniform etching. Another problem associated with wet etching is the isotropic nature of most wet etchants, which is problematic when the minimum pattern dimension of the substrate is comparable to the thickness of the material being etched. In that situation, it is hard to achieve precise pattern dimensions on a substrate.
Overall, the most commonly used wet etchants are hot alkaline etchants or acidic hydrogen peroxide (H.sub.2 O.sub.2) etchants. Typically, alkaline solutions remove organic films, while acidic solutions remove alkali ions, alkali compounds, and other metallic contaminants. For wet etching silicon (Si), mixtures of nitric acid (HNO.sub.3) and hydrofluoric acid (HF) are typically used. For wet etching silicon dioxide (SiO.sub.2), various HF solutions are typically used, usually further containing a buffering agent to prevent depletion of fluoride ions from the etchant during etching. Silicon nitride (Si.sub.3 N.sub.4) is typically wet-etched using a hot phosphoric acid (H.sub.3 PO.sub.4) solution. Aluminum (Al) is typically wet-etched using a mixture of phosphoric acid (H.sub.3 PO.sub.4), acetic acid (CH.sub.3 COOH), nitric acid (HNO.sub.3), and water (H.sub.2 O).
While wet etchants have many preferable characteristics as compared to dry etchants, dry etchants can be used in semiconductor fabrication without the need to dry the substrate being processed after an etching step. The added step of drying the substrate that is required when using a conventional wet etchant adds to the cost of semiconductor device fabrication. A lack of full process automation also results from the added step of drying the substrate. Another advantage of etching with a dry etchant is that it often decreases the safety hazards associated with wet etchants due to the relatively small amount of chemicals utilized in the dry etchant.
Supercritical fluids have been used to etch residue from a variety of surfaces or extract substances from various materials. A gas is determined to be in a supercritical state (and is referred to as a supercritical fluid) when it is subjected to a combination of pressure and temperature so that its density approaches that of a liquid (i.e., the liquid and gas state coexist). Supercritical fluids have been used to clean contact lenses by etching residue from lense surfaces, as disclosed by Bawa et al. in PCT Application Publication Number WO 95/20476. Supercritical fluids, namely carbon dioxide (CO.sub.2), have also been used to remove exposed organic photoresist films, as disclosed by Nishikawa et al. in U.S. Pat. No. 4,944,837, to form a patterned photoresist film. As further disclosed in Nishikawa et al., once an underlying layer is patterned by conventional methods, supercritical fluids are used to remove the patterned resist film.
Deionized water at elevated temperatures and pressures, but not supercritical temperatures and pressures, has also been used to etch thermally grown silicon dioxide (SiO.sub.2) on a silicon wafer. This was discussed in "Removal of Thermally Grown Silicon Dioxide Films Using Water at Elevated Temperature and Pressure," by Bakker et al., J. Electrochem. Soc., 142, 3940-44 (1995). The paper also discussed the solubility of quartz silicon dioxide (SiO.sub.2) in supercritical deionized water. Although Bakker et al. noted the use of supercritical water to promote solubility of quartz (i.e., a large-scale, uncontrolled process, as compared to etching in semiconductor fabrication as discussed in "A Portion of the System Silica-Water," by Kennedy, Econ. Geol., 45, 629, (1950)), Bakker et al. found that the preferred temperature and pressure for etching thermally grown silicon dioxide (SiO.sub.2) was below the critical point. Processing steps involved in the fabrication of semiconductor ICs need to be very controlled and precise in order to obtain the optimal density and electrical performance of the resulting IC. Thus, the processing steps utilized to solubilize quartz (SiO.sub.2), as compared to thermally grown silicon dioxide (SiO.sub.2), are not known to be readily interchangeable to one of ordinary skill in the art due to the differing process criteria. The quartz plates studied by Kennedy involved two contiguous square centimeters in solubilized surface area, while typical etched surface areas on semiconductor-based substrates are about one contiguous square micron or less. Furthermore, Kennedy noted that rough-ground or sawed crystals require a shorter time to solubilize as compared to smooth-lapped or well-polished crystals. Thus, due to the microscopically smooth nature of most layers utilized in semiconductor IC fabrication, it would not have been obvious to one of skill in the art to utilize supercritical water to etch inorganic layers within a semiconductor IC when increasing throughput is a prime fabrication concern.
An etching composition and method of etching various inorganic films in the fabrication of semiconductor ICs is needed that achieves an effective, highly uniform removal of inorganic material across a substrate. It is further desired that the etching composition and method of etching inorganic films be cost-effective and contribute to full process automation during semiconductor fabrication.